1. Field of the Invention
The present invention relates to a strapping machine, in particular, it relates to a strapping machine adapted to be capable of automatically or semi-automatically conducting a procedure-which includes looping a band made of a tape of a thermoplastic resin such as a polypropylene around an object to be strapped, and fusion-bonding overlapping end portions of the band to effect strapping.
2. Description of the Prior Art
To realize heightened efficiency and energy saving in strapping operation, a strapping machine has already been practically used in general which is adapted to be capable of automatically or semi-automatically conducting a procedure that includes looping a band made of a thermoplastic resin tape around an object to be strapped, tightening the band, fusion-bonding the resulting nodally overlapping end portions of the band under heat and pressure, and cutting the band. In other words, as already well-known, a strapping machine of this type is adapted to conduct a strapping procedure that includes holding, pressing, fusing and cutting a band and the like operation by rotationally driving a cam shaft and various cams fixedly fitted on the cam shaft by driving force from an electric motor to effect staggered actuations of mechanisms such as a press, heater plate and a slide table.
In such a conventional strapping machine, two-stage tightening is generally conducted as an operation for tightening a band, which comprises primary tightening for high-speed/low-torque low-torque tightening of a band and secondary tightening under low-speed/high-torque operation (see, for example, Japanese Examined Patent Publication No. 13205/1992). The transition from the primary tightening to the secondary tightening is effected by changeover of operation mode by means of a switch capable of sensing tension or the like means (see, for example, Japanese Unexamined Patent Publication No. 21304/1986).
Further, the applicant of the present invention has proposed a strapping machine which comprises stepping motors capable of rapidly and precisely controlling a rotational speed and a displacement angle according to number of pulses as a band feeding/tightening(feed/backfeed) motor and a cam driving motor, a resonance preventive damper mounted on an output shaft of the feed/backfeed motor, and a step-out sensor located between the damper and a motor body and utilizing a hall element and magnets to count number of pulses in a CPU (Japanese Patent Application No. 226189/1993). According to this proposal, number of motor driving pulses within an interval between pulses generated by passage of magnets over the hall element-located position is counted by means of the CPU. When the number exceeds a predetermined value (i.e.,tension in excess of predetermined one is exerted on a band), step-out is regarded as having taken place, thereby enabling changeover from primary tightening to secondary tightening to be effected automatically and smoothly. Further, vibration is damped and generation of noise is diminished.
In such a conventional strapping machine, however, there is a disadvantage that although fusion-bonding of a band is conducted with the band clamped by a rear clamp upon completion of the secondary tightening, the band tightened on purpose is likely to loosen and come off an object to be strapped during the period from the completion of the secondary tightening to a time point when the rear clamp becomes effectively in action. Accordingly, to prevent reverse rotation of the feed/backfeed motor in the feed direction, voltage application to the motor is required to be continued to hold the motor in a stopped state. This results in a problem of substantial power consumption for holding the motor in a stopped state, which is a disadvantage in terms of cost.